I'll tell you 'wat', I don't think the park know yet, but the most likely cause is an electrical fault as apparently there is a plant room not far from where the most extensive damage is.
I don't think it will be long until the Grand National is reopen to be honest. In that respect, Blackpool is one of the best and have already started some pretty heavy duty work.
I think a lot of it will be down to how many trains are servicable. If they're able to piece together two trains from what's left, I should think the ride will be open fairly soon, but if there isn't enough cars to work with, well, there could be quite a delay I suspect.
In summery: Don't know.